First off, I will be playing this Friday night NO MATTER WHAT!
Secondly, during D&D jr. I had a kid who was playing for the first time. I didn't really want him to play with us, but I was accomodating because he is my student and I have to be nice. I have to be nice, that is, until he starts fucking with my game.
Players were to sneak up on a goblin cave to grab an important item. I know, not a particularly original scenario, but the kids were new at this point, so I had to keep it simple. After initiative a few players take their turns and then I get to newbie. I ask him what he is going to do and he says, "You know what would be really cool Mr. Walker?" I don't even get to respond when he says, "If there weren't any goblins in the cave, but instead, a dragon LIVES THERE!"
The room is silent and I break it by saying, "well, since you are a level 1 fighter, I don't think you can handle a dragon just yet. Give it some time . . ."
"Or how cool would it be if there were a whole bunch of orcs in the caves and they came out to fight us?!"
Again, more silence, and I broke it with, "What is your character going to do?"
He ignores me and says, "Maybe I could write some stories for you Mr. Walker and we could play them out."
Completely angry at this point, I cut him off and say, "All right newbie, listen up, who is in charge of this game? You or me?" I don't let him answer, "I've been playing for a long time now, okay? For all intents and purposes, I am God here. I have established a story for you to play through and now you will play through it and you can make suggestions after the game, but while playing, please remember that this is your first round and you have no idea what you are doing. Again, I am God and you don't tell God what to do, he tells you what to do. Got it?"
He had to leave midway through the session because his parents found out what he was doing and he got in trouble for not telling them.
I guess this isn't a good enough representation of how I deal with suggestions. Normally, I take suggestions after a round and design the campaign on individual character stories. I don't take them during the rounds and I definitely don't take them from first time players.